Modern motorcycles generally have a rear suspension that includes a "swing arm" arrangement whereby the rear wheel is held for vertical travel about a pivot point located between the engine drive sprocket and the rear wheel impossible sprocket. Space requirements make it practically possible to pivot the swing arm at the axis of the drive sprocket. When the swing arm rotates about its pivot axis, the effective distance between the drive sprocket and driven sprocket shortens. This causes slack in the attached chain and varies the leverage moments on the swing arm due to abrupt change in chain tension on the tight side of the drive chain. The varying chain tension produced through the rotation of the swing arm due to horsepower transmitted to the rear wheel and shock loading due to chain slack, all act with varying moments upon the swing arm. This causes erratic action of the rear wheel suspension which is very undesirable for the rider. This situation occurs most frequently and is most noticeable during "moto-cross" racing wherein the motorcycle and rider are subjected to severe terrain at relatively high speeds. In such a situation it is of utmost importance that the rider be able to predict the behavior of the motorcycle suspension.
U.S. Pat. No. 1,143,793 granted to A. Strand on June 22, 1915 discloses a motorcycle with a chain drive arangement that is a partial solution to the above mentioned problems. Strand utilizes two individual chains interconnecting four sprockets. The engine drive sprocket is connected by a first chain to a first idler sprocket. The first idler sprocket is keyed to a shaft that also includes a second idler sprocket. The two idler sprockets are fixed relative to one another but are free to rotate with the swing arm to which they are mounted. The second idler sprocket is interconnected with the driven sprocket by the second endless chain. Since the sprockets are centered on the pivot for the swing arm, no chain slack wll develop as the rear wheel moves vertically about the pivot axis. As the driven sprocket moves vertically about the pivot axis, the chain is taken up on one side of the driven sprocket while it is let out in an equal amount from the opposite side. There is no resistance to pivotal movement of the swing arm due to tension in the chain flights. Furthermore, since the two idler sprockets are spaced apart, the forces operating through the transversely spaced pair of chains operate against one another to produce a vertical torque that, if significant amounts of horsepower are applied through the sprockets, would be very difficult to effectively control. A French Pat., 1,150,961, discloses an arrangement that is very similar to the Strand drive mechanism for centering the pivot axis of the swing arm with that of the sprocket that transmits driving power to the driven wheel sprocket.
U.S. Pat. No. 3,819,002 was grated to H. Heathwaite et al on June 25, 1974 for a drive system. In this arrangement, a bell crank is utilized having the driven sprocket at one end and an idler sprocket at a remaining end. The bell crank is privoted to a motorcycle frame intermediate the axes for the drive and driven sprockets. The idler sprocket is situated for engaging the lower or "slack" flight of the endless chain and moves in response to pivotal movement of the bell crank about its pivot axis. The idler sprocket supposedly takes up slack produced along the lower flight as the bell crank pivots about its axis. The idler sprocket is arrranged to engage the endless chain on an inside surface thereof such that when the rear end of the bell crank is pivoted upwardly, the idler sprocket will pivot downwardly, taking up the slack that would normally be created between the drive and driven sprockets on the "slack" side. It does not appear from the patent disclosure that slack resulting from downward wheel movement would be removed by the idler sprocket.
A United Kingdom Pat. No. 223,559 granted Mar. 12, 1925, discloses a rear suspension for a bicycle. This device shows a swing arm arangement that is adapted to be pivotably mounted to the rear fork of a bicycle frame. The arm mounts the wheel and drive sprocket on one side of the pivot and is connected to springs that oppose pivotal movement of the arm. An idler, take-up sprocket is also mounted to the swing arm between the pivot and drive sprocket to take chain slack as the rear wheel pivots about the swings arm pivot axis. This "take-up" sprocket engages the upper driving flight of the chain and is engaged with the lower, downwardly facing surface thereof. Only the springs attached to the free end of the swing arm serve the function of maintaining the rear wheel in a desired position relative to the bicycle frame.
The present invention utilizes a pair of spaced idler sprockets located on opposite sides of the pivot for the swing arm. These sprockets each engage the adjacent chain flight on an outside surface thereof. The sprockets are mounted to the swing arm to "pinch" the flights together at equidistant points from the pivot. By providing two freely rotatable idlers, we are able to hold the chain at a constant required length for all positions of the driven sprocket and, since power is applied through the idler sprockets on the swing arm, all moments on the swing arm due to chain tension are greatly reduced and a counterbalancing moment is applied in a direction opposite to the pivotal direction of the swing arm about its axis. There is therefore a tendency for the swing arm to seek and maintain a neutral position with the moments equalized about the swing arm pivot.